1. Field of the Invention
The invention relates in general to a flexible circuit board, and more particularly to a flexible circuit board fabricated by using photo-polymer.
2. Description of the Related Art
Flexible circuit boards used in the cartridge of an ink-jet printer serve as medium to lead the driving current to the chip for ink jetting. The driving current drives the cartridge and enables the cartridge to jet ink.
FIG. 1 shows a conventional flexible circuit board and its relative position to the dimple of the printer. Polyimide (PI) is a commonly used substrate 104 for the conventional flexible circuit board. Copper (Cu) and gold (Au) are two widely used materials for the conductive traces 106 in the flexible circuit board. The dimples 110 of the printer circuit contact the conductive traces 106 through holes 108 formed by tape automated bonding (TAB).
Etching and punching are two typical TAB manufacturing processes. Etching process is characterized by etching the tape while the punching process is characterized by punching the tape to form the holes.
FIGS. 2A˜2J illustrate the conventional etching process. On the substrate 202, such as polyimide (PI), a copper film 204 with a thickness of about 100 Å is formed by sputtering. On the bottom side of the substrate 202 and over the copper film 204, photo-resistors (PR) 206 are formed. After exposing and developing the PRs 206, the patterns of the holes and the conductive traces are defined. Next, as shown in FIG. 2F, on the side of substrate 202 with the exposed copper film 204, a copper layer 208 with several μm is plated. Then, as shown is FIG. 2G, the substrate 202 is etched to form holes 210 at the bottom side. The photo-resistors at both sides are then removed, as shown in FIG. 2H. Then, as shown in FIG. 2I, by a photolithography process, including steps of forming a photo-resister layer, exposing, developing and etching, the copper film 204 not covered by the copper layer 208 is removed. Finally, as shown in FIG. 2J, an insulation layer 212 is formed over the copper layer 208 for the purpose of protection.
The conventional etching process has the following drawbacks: time consuming, producing thick and sticky precipitate and large amount of wastewater, high cost and low yield rate.
FIGS. 3A to 3I show the conventional punching method to form holes on an insulation layer.
As shown in FIGS. 3A and 3B, an adhesive layer 304 is coated on the substrate 302. Then, the substrate 302 coated with the adhesive layer 304 is punched to form holes 306. Next, a copper layer 308 is adhered over the substrate 302 coated with the adhesive layer 304. Then, as shown in FIGS. 3E to 3H, a photo-resistor layer 310 is formed on the copper layer 308. After the photolithography process, including exposing, developing and etching, the pattern of the copper layer 308 is defined. Finally, as shown in FIG. 3I, an insulation adhesive layer 312 is formed on one side of the copper layer for the purpose of protection.
Compared with the etching process as mentioned before, this punching process is shorter in procedure, no problem of wastewater and lower cost. However, the intervals between each two holes are large and hard to reduce. So that, less holes can be formed in the same area, which therefore influences the precision contact between the printer and the TAB. Further more, the punching step could easily cause the breakage of the substrate and thus reduce the yield and increase the cost.